Irene Klotz

Scientists are stumped as to how a rock mysteriously appeared in images taken two weeks apart by NASA's Mars rover Opportunity.

The rover, which landed in an area known as Meridiani Planum a decade ago, is exploring the rim of a crater for signs of past water.

Another rover, Curiosity, touched down on the opposite side of the planet in 2012 for a more ambitious mission to look for past habitable environments.

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For the moment, though, scientists are pondering a more immediate question. On January 8, while preparing to use its robotic arm for science investigation, Opportunity sent back a picture of its work area.

Oddly, it showed a bright white rock, about the size of a doughnut, where only barren bedrock had appeared in a picture taken two weeks earlier. Scientists suspect the rock was flipped over by one of the rover's wheels.

It may also have been deposited after a meteorite landed nearby.

Either way, the rock, dubbed "Pinnacle Island" is providing an unexpected bonus for scientists.

"Much of the rock is bright-toned, nearly white," NASA said.

"A portion is deep red in colour. Pinnacle Island may have been flipped upside-down when a wheel dislodged it, providing an unusual circumstance for examining the underside of a Martian rock."

Despite the perfectly reasonable explanation, its left many thinking it may not be the only one.

"As to how it got there, well, on Sunday I thought I was on to something when, colourising some images of the surrounding area, I thought I spotted what looked like trails leading away from a couple of small rocks," he wrote.

"If they were real, and not either just imaging artefacts or my own over-active imagination seeing things that aren't really there, could they be a sign that there has been a more general movement of rocks in this area recently, which might make the ejecta theory more plausible?"